218 PALEONTOLOGY OF ILLINOIS. 



sharply rounded anterior angle is about 11.5 mm., the basal rim 

 somewhat expanded laterally and produced posteriorly, increasing the 

 length of the border to about 13 mm. The transverse diameter 

 across the posterior margin exceeded 13 millimetres ; in consequence 

 of the mutilation of the inner articular border the entire breadth of 

 the tooth is not shown. The posterior margin is somewhat deeply 

 concave in outline, the anterior extremity was apparently gently 

 arched or nearly straight, but it bears evidences of wear by which 

 its outline is made to appear slightly concave, the coronal margins 

 well denned from the base, making a slight angulation in front and 

 behind sharply defining the inbeveled inferior basal face. 



The above described tooth is peculiar in outline, but the coronal 

 sculpturing resembles that of Psammodus glyptus, also P. reticulatus, 

 N. and W., of the Chester limestone, although it cannot be con- 

 founded with either of those species. The rugae show irregular dis- 

 position, in which particular the tooth contrasts with the superficial 

 ornamentation observed in individuals of the form referred to P. 

 crassidens, while it represents a different form from that upon which 

 the latter species was based. 



Geological position and locality: St. Louis formation, Pella, Iowa. 



PSAMMODUS CRASSIDENS, St. J. and W. 



PI. XVIII, Fig. 2-6. 



Psammodus rugosus, NEWBERRY and WORTHEN, 18G6, Illinois Geol. Surv., II, p. 108, PI. XI, 

 f. 3, 3 a. Not P. rugosus, Agassiz. 



Teeth attain large size, subrhomboidal in outline. Coronal surface 

 moderately arched from behind forwards, somewhat steeply sloped 

 in the region of the anterior margin, with a distinct longitudinal 

 median depression, bordered on either side by a moderately promi- 

 nent fold rounded to the lateral borders, where the coronal enamel 

 is more or less distinctly defined from the basal portion of the 

 tooth. The outer border is gently arched, rather sharply rounded 

 into the anterior margin, terminating posteriorly in an acute angle ; 

 anterior margin also broadly arched from the rounded outer angle 

 to the subacute inner angle, presenting in unworn individuals a 

 sharp edge inferiorly inbeveled with the basal portion; posterior 

 margin closely repeats the curvature of the opposite extremity, 

 sometimes interrupted by undulations, and terminating in sharp 

 angles; the inner border conforms in a general way to the curva- 

 ture of the opposite side, but toward the anterior extremity the 



